Did you remove the starter when you replaced the solenoid? Has the starter been removed sine you bought the boat? Did you know that the Bendix gear and shaft are supposed to be lubricated annually?

If the Bendix shaft rusts, and I will assume it has because of the rust I saw on the head in the photo of the solenoid, the gear will bind when it cranks. You won't have any idea when, until it does, either.